This relates generally to testing equipment used in manufacturing electronic devices, and more particularly, to self-calibrating test systems used during device manufacturing.
Electronic devices such as cellular telephones include numerous electronic and mechanical components. Tests are performed during manufacturing to ensure that devices are operating satisfactorily before they are shipped and sold to end users. For example, pass-fail tests are often performed in which a device is tested to determine whether it is operating within specified limits. This type of test may be performed on the wireless circuitry in a device. If a device is not operating properly, the device can be reworked or discarded.
Test equipment such as radio-frequency (RF) test stations can become unreliable if not periodically calibrated.
Each test station typically includes a radio-frequency test unit that generates and receives radio-frequency signals from a device under test (DUT). A test host that is coupled to the test unit is used in controlling operation of the test station. A radio-frequency test chamber may be included in the test station. During tests, a device under test may be mounted on a fixture in the test chamber. A cable may be used to couple the test unit to an antenna in the test chamber. This allows the test station to be used in performing over-the-air (wireless) tests. The test unit may also be connected to a DUT using a cable and the fixture in the test chamber.
One way to calibrate an RF test station involves measuring test station performance using equipment such as a vector network analyzer or power meter. With this type of arrangement, a technician must typically remove all test cables and other components that are located between the test unit and the DUT. The technician must then measure the RF performance of these components using the vector network analyzer or power meter. The measurements may be used to determine how much loss (RF attenuation) is being produced by the cables and other components. The loss measurement may be used to calibrate the test station.
Another way to calibrate an RF test station involves use of a reference DUT. The reference DUT can be calibrated to known standards. Once calibrated, the reference DUT may be used to calibrate an RF test station by comparing measurements made using the reference DUT and the test station to expected values. Both wired and wireless path loss measurements may be made using a reference DUT.
Calibration operations can also be made by placing a reference antenna in a test chamber and by feeding the reference antenna with a signal from a signal generator.
Calibration techniques such as these tend to be manpower intensive. For example, it may take a technician a significant amount of time to calibrate a single station. To perform routine calibration operations in a large factory, numerous technicians must be available. The possibility of human error is also a factor when using these calibration techniques. Cabling must be detached and reattached, leading to the potential for poor cable attachment and random measurement errors. In schemes that rely on reference DUTs, there is a possibility that the reference DUTs will become damaged or lost. The process of producing and distributing reference DUTs may also be burdensome, particularly in an organization with geographically dispersed manufacturing facilities. Schemes that use reference antennas may be sensitive to antenna placement due to the near-field nature of this type of calibration measurement.
These techniques also suffer from the lack of a single absolute external reference for all of the test stations. Satisfactory calibration of a large number of test stations may therefore involve the use of numerous vector network analyzers, reference DUTs, or reference antennas. Large amounts of calibration equipment can be expensive to maintain and can be prone to loss and damage.
It would therefore be desirable to be able to provide improved ways in which to calibrate test equipment such as radio-frequency test equipment used during manufacturing of electronic devices.